1. Field to the Invention
The present invention relates to a golf club shaft. More particularly, the present invention relates to a golf club shaft composed of a laminate of a plurality of prepregs lightweight and having a high flexural strength and impact strength to prevent it from being broken.
2. Description of the Related Art
A golf club shaft composed of a plurality of prepregs containing a resin and a carbon fiber impregnated with the resin is widely used because the prepregs are lightweight and have a high strength and vibration-damping performance. The prepregs are molded by winding them around a molding mandrel to form the shaft.
Once many golfers played golf to compete in a race but nowadays want to play golf as leisure. Therefore there is a growing demand for a golf club that can be handled easily and for a golf club shaft that is lighter and softer.
To make the shaft lightweight, it is necessary to decrease the amount of carbon fiber to be used. Decreasing the amount of the carbon fiber causes the strength of the shaft to be lower. To allow the shaft to have an appropriate degree of rigidity, it is necessary to use the carbon fiber having a high modulus of elasticity. The carbon fiber having a high modulus of elasticity has a low elongation percentage (deformation amount before breakage). Thus as the modulus of elasticity of the shaft becomes higher, the strength, in particular, the compression strength thereof becomes decreasingly low. Even though the tensile modulus of elasticity of the shaft is increased, the compression strength thereof does not become high.
In the case where the shaft is allowed to have an appropriate degree of rigidity, the strength of the shaft, particularly, the compression strength thereof becomes low. Consequently the shaft becomes frail. When a flexural load and an impact load are applied to the shaft, the shaft is liable to be broken.
To overcome the above-described problems, various proposals of golf club shafts are made as being lightweight and having a high rigidity.
For example, the golf club shaft composed of carbon fiber reinforced resin is disclosed in Japanese Patent Application Laid-Open No. 5-49717. The golf club shaft has the inner layer composed of the angular layer inclining to the axis thereof and the outer layer composed of the inner and outer straight layers (two-layer structure) parallel with the axis thereof. The golf club shaft further includes the reinforcing layer disposed at the tip where the head is mounted and grip thereof. The carbon fiber of the inner straight layer of the two-layer structure has a high elasticity and strength, whereas the carbon fiber of the outer straight layer thereof has a high strength.
The golf club shaft disclosed in Japanese Patent Application Laid-Open No. 6-165844 has the compressive angular layer and the tensile angular layer. In the compressive angular layer inclining at an angle in a clockwise direction to the axis thereof, the PAN based carbon fiber having a high compressive modulus is used. In the tensile angular layer inclining at the angle in a counterclockwise direction to the axis thereof, pitch based carbon fiber having a high tensile modulus of elasticity is used.
The golf club shaft disclosed in Japanese Patent Application Laid-Open No. 2001-62014 is made of a composite prepreg partly containing an organic fiber combined with a carbon fiber having a low tensile modulus of elasticity of less than 1500 Pa.
In the golf club shaft disclosed in Japanese Patent Application Laid-Open No. 5-49717, the carbon fiber of the outer straight layer of the two-layer structure has a high strength, whereas the carbon fiber of the inner straight layer thereof is highly elastic. Therefore the golf club shaft is frail and easily broken.
In the golf club shaft disclosed in Japanese Patent Application Laid-Open No. 6-165844, the highly elastic carbon fiber is used. Thus the golf club shaft is also frail and easily broken.
In the golf club shaft disclosed in Japanese Patent Application Laid-Open No. 2001-62014, the specific composite prepreg containing the organic polymerized fiber combined with the carbon fiber having a low tensile modulus of elasticity. Thus the golf club shaft costs high. Further both the organic polymerized fiber and the carbon fiber having a low tensile modulus of elasticity have a comparatively high breaking elongation percentage but have a low tensile strength (tensile strength of organic polymerized fiber is about 3000 MPa, and tensile strength of low-modulus carbon fiber is in the range of 1100 to 2400 MPa). Therefore the composite prepreg is effective for improving the impact strength of the golf club shaft but does not allow it to have a sufficient flexural strength.